1. Field of the Invention
The present invention relates generally to medical devices, and more specifically relates to implantable electronic devices for muscle stimulation.
2. Description of the Prior Art
The earliest implantable pacing systems operate asynchronously to normal physiologic functions. U.S. Pat. No. 3,057,356, issued to Greatbatch, teaches such a pacer which has a fixed rate oscillator coupled to an output driver circuit. Each cycle of the fixed rate oscillator causes generation of a stimulating pulse by the output driver circuit. Designs soon incorporated the demand feature which senses natural electrical activity in the heart and generates a stimulating pulse only if none is provided physiologically within a fixed escapement period. Demand mode pacers are now the most popular and probably outnumber all other types combined. Even though later improvements provide programmability of the escape interval, the effective pacing rate of such demand pacers is non-responsive to changes in physiological requirements.
There has been considerable work done in the area of physiologically controlled pacers. Most of these devices measure some parameter and adjust the period of the oscillator in response to changes therein. An early such device is taught by Cohen in U.S. Pat. No. 3,358,690. In this special case, however, the physiologic parameter measured is instantaneous blood pressure within the right atrium. This system will apparently work well for patients having complete atrial ventricular block with properly paced atria. The Cohen approach is not likely to be effective for sick sinus syndrome, sinus/atrial block, or similar disorders.
Later physiologically controlled pacer systems have been developed which have more general application and may be categorized by the parameter measured. Krasner et al. in U.S. Pat. No. 3,593,718 teaches sensing of mechanical activity within the thorax. It is assumed that changes in respiration rate are thereby sensed. The oscillator of the pacing system has its rate controlled by changes in this parameter. Dahl, in U.S. Pat. No. 4,140,132 teaches an improved implantable sensor for determining level of a patient's physical activity for rate-controlling a pacer as taught by Krasner et al.
Bozal Gonzalez in U.S. Pat. No. 4,201,219 teaches rate control of a pacer based upon neurological activity. Electrodes imbedded in the nervous system sense electrical activity. Somehow, the amount of this electrical activity is used to control the oscillator rate of an asynchronous pacer.
By far the most promising techniques appear to involve the sensing of certain chemical parameters of venous, often intracardiac blood. Alcidi in U.S. Pat. No. 4,009,721 teaches a pacer controlled by the pH of the blood. A chronically implantable sensor determines the blood pH. The rate of an oscillator of an asynchronous pacer is controlled by the sensed pH. It has been shown that blood pH decreases during prolonged muscular exercise. Mauer et al. in U.S. Pat. No. 4,252,124 teach an improved pH sensor.
Wirtzfeld et al. in U.S. Pat. No. 4,202,339 teach a pacing system having the rate of an asynchronous oscillator controlled by the 0.sub.2 level of the intracardiac venous blood. As with all known prior art, physiologically controlled pacers, Wirtzfeld et al. teach what is essentially an asychronous pacer as described by the above identified patent issued to Greatbatch, with an oscillator rate directly controlled by the measured parameter. As such, these devices are less than optimal for treating cases of partial heart block.